With a reciprocating piston engine operating on a two stroke cycle, part of the compression and expansion strokes are used for the purpose of exhaust and induction. It is common for the piston itself to be used to open and close inlet and exhaust ports. On upward movement of the piston, the crankcase is decompressed and a charge of air or combustible mixture is admitted into the crankcase through a self-acting valve. During the expansion stroke the charge of air or combustible mixture in the crankcase is compressed, and near the end of this stroke the exhaust port is uncovered to allow the combustion products to discharge from the combustion chamber. Further movement of the piston uncovers the inlet port and the compressed charge from the crankcase flows into the combustion chamber. The exhaust and inlet ports are open simultaneously for a short period so that the incoming charge can assist in clearing the combustion chamber of combustion products. The cycle is completed by compression and subsequent ignition of the charge trapped in the combustion chamber.
A deficiency of this common arrangement in two-stroke engines is that the process of clearing the combustion products from the combustion chamber and recharging it is restricted, owing to the little time available for induction and exhaust and the fact that for a portion of this time the inlet and exhaust ports are both open, with the result that some incoming charge escapes with the exhausting combustion products.